1. Field of the Invention
This invention relates to a circuit protection arrangement comprising conductive polymer devices, and to circuits and assemblies comprising such an arrangement.
2. Introduction to the Invention
Electrical devices exhibiting a positive temperature coefficient of resistance (PTC behavior) are well-known for use as circuit protection devices. Such devices generally comprise a PTC element composed of a conductive polymer composition in which a particulate conductive filler is dispersed in a crystalline polymeric component. First and second electrodes, e.g. in the form of metal sheets, are attached to the conductive polymer to allow electrical connection to the device in the circuit. During normal operation the PTC device is in a low resistance, low temperature condition. When a very high current condition occurs, for example, due to a short circuit or an overvoltage condition, or a very high temperature condition occurs, for example, during excessive charging in a battery application, the device “switches” or “trips” into a high resistance, high temperature condition, thus decreasing the current through the circuit to a low level and protecting the electrical components in the circuit.
The temperature at which this transition from low resistance to high resistance occurs is the switching temperature, Ts. Ts is defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of the PTC element as a function of the temperature which lie on either side of the portion of the curve showing a sharp change in slope. The switching temperature is a function of the type of conductive polymer composition used in the device. In general, Ts is slightly lower than the melting temperature Tm of the polymeric component, where Tm is defined as the peak of the endotherm of a differential scanning calorimeter. When there is more than one peak, Tm is defined as the temperature of the peak having the greatest volume, i.e. the majority component in the composition. Therefore, depending on the application and the anticipated thermal environment, a PTC device based on a particular composition having a particular Ts is selected.
PTC devices are rated based on their ability to withstand a particular combination of voltage and current without failing. Thus a PTC device has a hold current, IH, which is the largest steady-state current that, under specified ambient conditions, can be passed through a device without causing the device to trip into a high resistance state; a trip current, IT, which is the smallest steady state current that, if passed through the device, will cause the device to trip; and a maximum interrupt current, Imax, which is the highest fault current that can be safely used to trip a device under specified conditions. The hold and trip currents are a function of temperature, decreasing with increasing temperature.